"A well regulated militia being necessary to the security of a free State, the right of the People to keep and bear arms shall not be infringed."
What people on both sides don't understand is the first part. What this means is that INSTEAD of having a standing Army which the colonists lived in fear of and was required to enforce tyranny, it was necessary to have a well regulated (armed and trained) militia. If you want to be free and secure this is the only option. If you have a standing Army you will be secure but not free. If the the citizens well armed and trained you won't have security.
The best example is the modern world is Switzerland. Everyone is well trained and armed. But the professional full timers in the army are only about 5% while the rest are the militia.
This view is backed up by the Powers of Congress which make a difference between the Navy (which was intended to be permanent) and the Army (which was only funded for 2 years at a time and only called up from the militia when needed).